var
  v: Variant;
  HTMLDocument: IHTMLDocument2;
begin
  HTMLDocument := WebBrowser1.Document as IHTMLDocument2;
  v := VarArrayCreate([0, 0], varVariant);
  v[0] := HTMLString; // this is your html string
  HTMLDocument.Write(PSafeArray(TVarData(v).VArray));
  HTMLDocument.Close;
  ...
end;